Disk usage
The bottom of each panel says what remains free on the displayed volume. A drive’s button, in the bar, carries its pie chart.
See what weighs the most
“Disk usage”, in a folder’s menu, draws what weighs the most: each area is a subfolder, its surface is its size, a click goes down into it. It is what you open when a disk fills up and you do not know with what.

For a single folder, Space is enough: it measures it and writes its size in the column.
After a deletion
A deletion gives the space back and the pie chart says so as soon as you hover over it — except through the recycle bin, which keeps the bytes: they only come back when it is emptied.
Meneaufile manual, in English.